A permanent magnet synchronous motor, as using permanent magnets as a rotor thereof, can provide high output and high efficiency in comparison with an induction machine and other motors. As a stator, punched out silicon steel sheets are used in a laminated manner, and coils are wound around the silicon steel sheets. Magnetic flux, generated from the permanent magnets of the rotor, provides interlinkage with the coils, and thereby, induced voltage is generated at the coils. By applying voltage higher than the induced voltage from an inverter connected with the coils, and applying electric current synchronized with the rotation through the coils, the rotor generates torque.
At this moment, the induced voltage increases according to rotating speed. Because of this, when the induced voltage becomes higher than the voltage of the inverter, it is impossible to apply the electric current through the coils. Then, as the motor cannot generate torque, the upper limit of the rotating speed reaches a ceiling. In order to increase the upper limit of the rotating speed, known is the following prior art for weakening the magnetic flux of the magnets or reducing the magnetic flux providing interlinkage with the coils.
There is a control method called as the field-weakening control where magnetic flux, opposed to the magnetic flux generated from the magnets, is made to generate from the coils so that the magnetic flux from the magnets does not provide interlinkage with the coils. When using the field-weakening control, the magnetic flux of the magnets looks weakened. Then, the induced voltage decreases, and the rotating speed increases.
There is a structure called as the field-alternative structure where two kinds of magnets which have different force of holding magnetic flux of a magnet (the coercive force) from each other are arranged to the rotor, and by electric current from outside the direction of magnetic flux of magnets having weaker coercive force is made counter to the direction of magnetic flux of magnets having stronger coercive force. By this structure, the polar character of the magnets having weaker coercive force is inverted, and the magnetic flux output from the magnets having stronger coercive force enters into the magnets having weaker coercive force. Thereby, the magnetic flux which provides interlinkage with the coils is reduced.
The patent literatures 1 and 2 disclose that the coils wound around the stator are divided into two groups, and when the rotating speed is low, the two groups are connected in series and are used, and when the rotating speed is high, the two groups of coils are disconnected from each other so that only one of the two groups is used. Thereby, it is possible to fall the magnetic flux providing interlinkage with the coils to the one-half.
In the patent literature 3, it is disclosed that an art that the coils wound around the stator are divided into two groups, and when the rotating speed is high and the torque is low, winding units are connected in parallel, and when the rotating speed is low and the torque is high, the winding units are connected in series, so that the magnetic flux providing interlinkage with the coils is fallen to one-half.
In the patent literature 4, it is disclosed that by making the magnets arranged to the rotor rotatable, when the rotating speed is low, the magnets are arranged in the direction opposed to the coils, and when the rotating speed is high, the magnets are arranged in the direction parallel to the coils. The magnets which are arranged in the direction parallel to the coils the area of the magnets is small as viewed from the coils. Because of this, it is possible to reduce the magnetic flux providing interlinkage with the coils.